In general, when a lubricating base oil is produced from crude oil, the crude oil is first subjected to atmospheric distillation, and the resulting residual oil is further subjected to vacuum distillation to separate various lubricating oil fractions having varied viscosities and vacuum distillation residual oil. The vacuum distillation residual oil is subjected to solvent deasphalting, thereby removing asphalt contents and obtaining a heavy lubricating oil fraction (bright stock). These lubricating oil fractions having varied viscosities, including the bright stock, are further subjected to solvent refining, hydrofinishing, dewaxing and the like steps to produce the lubricating base oil of interest.
On the other hand, a hydrocracking process is known as a process for the production of a lubricating base oil having a high viscosity index. In this process, heavy oils such as vacuum gas oil fraction, bright stock and the like are subjected to hydrocracking under high temperature and high pressure conditions in the presence of a catalyst, and a high viscosity index base oil is produced from the resulting oil.
Examples of the hydrocracking of heavy oil are disclosed, for instance, in JP-B-46-3267, JP-B-50-26561, JP-B-50-36442, JP-B-51-15046, JP-B-51-41641, JP-B-54-21205, JP-B-54-31002, JP-B-57-17912, JP-B-62-5958, JP-A-48-49804, JP-A-63-258984, JP-A-64-6094, JP-A-3-197594, JP-A-3-223393 and the like. (The term "JP-B" as used herein means an "examined Japanese patent publication", and the term "JP-A" as used herein means an "unexamined published Japanese patent application".) Also, hydrocracking and isomerization of wax and the like as the stock oil are disclosed, for instance, in JP-B-57-17037, JP-B-60-22039, JP-A-50-92905, JP-A-51-146502, JP-A-52-136203, JP-A-1-223196, JP-A-1-301790, JP-B-4-503371, JP-A-4-226594, U.S. Pat. No. 4,547,283, U.S. Pat. No. 4,906,350, EP-A1-0464547 and the like.
Development of a low viscosity base oil having a high viscosity index has been called for in the area of not only engine oil but also hydraulic fluid for construction machine use.
However, production of a low viscosity lubricating base oil having a high viscosity index is not easy because, when it is produced by the solvent refining process in the art, the product is limited to certain lubricating oil fractions from specific high quality crude oil, and an extremely high extractant ratio is required in the solvent refining step.
Because of this, the aforementioned hydrocracking process has been developed and put into practical use only as a process for the production of low viscosity lubricating base oils having high viscosity index from mineral oil materials. However, since heavy oils such as vacuum gas oil fraction, bright stocks and the like are used as the stock oil in the hydrocracking process in the art, the viscosity index of the lubricating oil fraction produced by this process is high in the case of a distillate having a relatively high viscosity, but the index is not so high when the fraction has a relatively low viscosity of 3.0 to 5.0 mm.sup.2 /s as a kinematic viscosity at 100.degree. C.
In consequence, the hydrocracking process in the art aims at producing a lubricating base oil having a relatively high viscosity and, therefore, is not suitable for the production of a lubricating base oil having a relatively low viscosity and a high viscosity index.